In recent years, image quality of color light-sensitive materials for photographic use has been improved so much that, in obtaining prints of about service size from 35 mm film, fairly satisfactory graininess and sharpness have been attained.
However, in the case of reducing the film size to, for example, 110-size, disc size, etc., the image quality is still so unsatisfactory with respect to graininess and sharpness that, in consideration of small format photographing materials, improvement of image quality is still an important object with respect to color photography.
As to improvement of sharpness, there are basically two types of techniques; one being a technique of strengthening edge effect using mainly development inhibitor releasing (hereinafter DIR) couplers; the other being a technique of reducing scattering of light in emulsion layers, which is mainly attained by reducing the thickness of the emulsion layers.
Of these, the technique of using DIR couplers is effective for improving sharpness in a comparatively low frequency region of MTF curve (MTF curve being described in The Theory of the Photographic Process, 3rd Ed. (compiled by T. H. James and published by Macmillan Co.), p. 536). In consideration of enlarging a small format photographic material, improvement of MTF in a comparatively high frequency region is of importance as described in U.S. Pat. No. 4,500,634 and Japanese Patent Application (OPI) No. 36249/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). In order to attain the improvement in a high frequency region, a great reduction in the thickness of emulsion layers is necessary as has been described hereinbefore. As to techniques for reducing the thickness, various techniques are disclosed in the aforesaid published application, which discloses that the use of a polymer coupler, i.e., a coupler having a polymerizing moiety, is effective.
In general, blue-sensitive layers are preferably provided as uppermost layers in photographing materials in view of color reproduction, etc. Therefore, it is of extreme importance to reduce the thickness of the blue-sensitive layer.
Processes for adding a polymer coupler to a hydrophilic colloidal medium to disperse the coupler in the medium are basically grouped into two types. One type is a process of forming latex particles by emulsion polymerization as described in U.S. Pat. Nos. 3,370,952, 4,080,211 and 3,451,820, and adding them to an emulsion, or a process of dissolving in a solvent a polymer coupler obtained by solution polymerization, emulsifying and dispersing the resulting solution, and adding the dispersion to an emulsion.
The other type is a process of introducing a water-soluble group (a hydrophilic group) into a polymer coupler to render the coupler water-soluble, and adding the polymer coupler to a hydrophilic colloidal medium. In this case, the polymer coupler is considered to be intimately mixed with the hydrophilic colloidal medium without forming a latex.
In this specification, the former polymer couplers are referred to as oil-soluble polymer couplers since they have an extremely small solubility in water and are readily soluble in high boiling organic solvents, and the latter polymer couplers as water-soluble couplers since they are water-soluble.
Oil-soluble polymer couplers can contain color-forming units in a high density and can be applied using only a small amount of, or no, high boiling organic solvents, thus enabling reduction in the thickness of the emulsion layer. In Japanese Patent Application (OPI) No. 73151/86, an oil-soluble dye-diffusion type yellow polymer coupler latex is disclosed.
However, they have a serious defect in that, as the amount of hydrophilic colloidal medium (gelatin being often used) is reduced for reducing the thickness of the emulsion layer, film strength becomes weak since oil-soluble polymer couplers are soft in comparison to gelatin. Even if the above polymer coupler latex is used, reduction in the film strength cannot be avoidable since the polymer coupler itself is oil-soluble.
On the other hand, the water-soluble polymers can be coexistent with gelatin and, if necessary, a hardener capable of forming a network structure with gelatin or a group capable of reacting with such hardener may be introduced thereto, and hence the water-soluble polymer couplers in principle permit reduction in the thickness without sacrificing film strength in comparison with the oil-soluble polymer couplers.
As yellow dye-forming water-soluble polymer couplers, U.S. Pat. Nos. 3,211,552, 3,299,013 and 3,424,583 and Japanese Patent Publication Nos. 18466/61, 13375/69 and 16184/68 describe 4-equivalent yellow dye-forming water-soluble polymer couplers, and U.S. Pat. Nos. 4,207,109 and 4,215,195 describe 2-equivalent yellow dye-forming water-soluble polymer couplers. However, these water-soluble polymer couplers have been found to have a serious defect that the dark heat fading properties under wet heat conditions after color development are deteriorated.